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This article could save you thousands of Rands per year! Supermarkets and pharmacies have hordes of batteries on display at the checkout – and most people take a pack of alkalines because they are well 'branded' and advertised, regardless of what they are going to be used for. If you can't be bothered with the text, then jump to the summary at the end and deal with the details later.

Safety first: Batteries contain dangerous chemicals. They also contain a huge amount of energy. Please observe safety tips. I am not responsible if you poison yourself or blow yourself up.

Batteries, i.e. Dry Cells, Accumulators and rechargeables represent portable power and convenience, generally at a high cost compared to mains electricity. In books on batteries, the term Primary Battery is used to refer to non-rechargeable and Secondary Battery to rechargeable cells. I'm going to mention some archaic batteries because you may encounter references to them in old instruction manuals and wonder what they were. Whilst on the subject of archaic batteries – we might as well see how they were invented.

A Brief History of Batteries

The earliest known batteries may have been the “Baghdad batteries” found in Iraq. They consisted of an iron rod protruding through an asphalt lid on an earthenware jar. Inside the jar was a copper cylinder. These date from 250 A.D. and we can't be certain that they were indeed batteries.
The first battery cell is considered to be the Galvanic Cell, which was an arrangement of silver and zinc disks with a blotter soaked in salt water in between them. In 1800, Alessandro Volta made an arrangement of a series of Galvanic cells, to produce the “Voltaic Pile”.

In 1820, John Frederic Daniell invented an improved voltaic cell which was used in early telegraphy systems. This used a zinc plate immersed in zinc sulphate and a copper plate immersed in copper sulphate, with a salt water “bridge” connecting the two systems. This was later improved by making a zinc inner electrode inside a porous cup which was in turn inside a copper cylinder. The whole assembly being inside a glass jar. The porous cup kept the copper and zinc sulphates apart. Electrons were produced at the zinc anode (-) and absorbed at the copper cathode (+). When the battery became “dead” - it was a matter of taking it apart, cleaning the deposits off the copper and zinc electrodes and replacing the copper and zinc sulphate electrolytes.

It wasn't until 1866 that Georges Leclanché invented the forerunner of the modern dry cell. This was the original carbon zinc battery. It consisted of a glass jar, containing a zinc cylinder surrounding a carbon rod. The electrolyte being sal ammoniac (ammonium chloride). These cells had a tendency to polarise – lose power – because hydrogen bubbles formed on the electrodes. Manganese dioxide was added round the carbon cathode to prevent this. Later on, the glass jar was replaced by a zinc container to produce a “recognizable” wet-cell battery. “D size” (see later) dry batteries were produced in 1896.

At about the same time, other inventors were working on rechargeable batteries. I will deal with the history of these and later types of battery when I go into greater detail of each type that you can purchase today. The fact that today's dry cells and car batteries were invented over a century ago may lead you to suppose that battery development has become “frozen in time”, but in fact, a huge amount of effort is being put into battery development, spurred on by the need for portable power for consumer devices.

Battery Installation

Whether you are replacing your transistor batteries or installing a UPS, there are some basic basics to remember:

Switch off!

Take heed of the battery manufacturer's instructions.

Ensure that you have the correct voltage and battery type. Some transistor radios won't work with the wrong battery (NiCad/NiMH) because the voltage is too low.

Clean and polish the battery connectors and battery terminals to make sure you have a good connection. Putting petroleum jelly on the battery terminals helps prevent corrosion.

Check you have the batteries correctly inserted so that the polarity is correct. Now check again before you switch on. Modern LED torches require that you have the correct polarity.

On UPS and inverter systems, install a fuse or circuit breaker in series with the battery.

Do not try wiring AC sources (generators, inverters in series or parallel).

Tip: Use a fuse or circuit breaker beteen the positive connection and the load. Some inverters have this already, so check first.

Principle of Operation

Batteries are devices that work through the action of a chemical reaction that takes place between two dissimilar metals through the medium of an electrolyte. In early cells, electrolytes were a liquid – usually a solution of a metallic compound. Nowadays, they are often in the form of a jelly. Let's look at the Daniell cell, referred to above:

Diagram of a Daniell Cell

What happens is that the zinc anode loses some of its metal to the zinc sulphate solution in the form of positive ions (an ion is an atom with a net electric charge.) and has some electrons to spare, which travel along the electrical circuit to the copper. The copper cathode, on the other hand, gains copper and absorbs the electrons from the circuit. It can do this by gaining positive ions from the solution, so that everything remains in balance. The movement of the electrons gives rise to an elecromotive force, which we can measure on a voltmeter.

The batteries to be described use different materials and different chemistry, but the essential principle remains the same. In the case of the Daniell cell, the chemical reactions can not be reversed. In rechargeable batteries, the reactions are reversed recharge.

Note that, confusingly, we tend to think of electricity flowing from positive to negative – this is the opposite way to the way electrons travel. The flow from positive to negative is a convention, which works in practice.

Electric Power - Calculation and Measurement

Most of us are intuitively familiar with the terms applied to the measurement of power. We talk of a high performance motor car with an engine that can produce 300 kW or 400 brake horse power (bhp). We look at a 100 Watt light bulb and know that it produces more light than a 60 Watt globe. We routinely use “power” to compare the performance of consumer objects, such as light bulbs, motor cars and audio systems.

To get a better idea of some of the concepts, let us examine the following simple circuit:

Measuring power delivered to resistor R

Power is measured in “Watts”.Note that in the SI (Systeme Internationale), units named after people are given a capital letter, thus J= Joules, W=Watts, T=Tesla and so on).

For the purposes of our discussion, the amount of energy that an elecrical load uses in Watts, is given by the formula :

W=I\times V

$W$ is the power consumed
$I$ is the current flowing through the circuit in Amps, and
$V$ is the electromotive force measured in Volts

Since Ohm's law states:

V=I\times R

Where:
$R$ is the resistance of the circuit, measured in Ohms
then the power dissipated in a resisive load is given by :

{I^2}R

or

{V^2}\over R

So, if you have a 100 Ohm resistor with one hundred volts across it, then the current flowing through it will be 1.0 Amps, and it will dissipate 100 Watts of energy (in the form of a great deal of heat). When we connect a resistor across a battery in this way, we refer to it as the load resistor. This is similar to a truck expending power when it hauls a load.

A related question, in the case of batteries, is “how long can it keep supplying power ?”. This is measured in Watt-hours, or kilowatt-hours and is really what you pay for when you purchase a battery.

For example, if we look at a rechargeable nickel metal-hydride battery with a capacity of 2 Amp-Hours (It will have 2000mAh printed on the cover) and a terminal voltage of 1.25 volts, then (on the face of it), it can deliver 2.5 Watts for 1 hour, or 2.5 Watt-hours. But here, a complication sets in. As you draw more and more current from a battery, its capacity becomes less and less. Batteries are often specified as having a certain capacity at a 20 Hour rate. For example, a 100 Amp-Hour battery will let you draw 5 Amps for 20 Hours, but if you draw 50 Amps - you certainly will not be able to draw that amount of power for 2 Hours. If you are lucky, the battery will last for maybe 45 minute to an hour, and as it ages, the capacity will be less and less. See Appendix.

The other thing might be of interest, is the “energy density” - how much power have we can pack into a given volume, or how much power we have for a given weight. We can express this in Watt-hours per litre if we are concerned with size, or Watt-hours per kilogram if the weight concerns us.

Battery Circuit – Internal Resistance

If we look at the circuit for a simple flashlamp consisting of a battery, a torch globe and a switch all wired in series, we get a circuit like the one below:

Simple Flashlamp Circuit Showing Internal Resistance of Battery: Rb

As you can see, I added $R_b$. This is a low value resistor that represents the fact that a battery is made up of components, such as the electrolyte that has resistance. If you short circuit a battery by connecting the terminals together, all the battery's energy is delivered to the battery itself. (i.e. $R_b$). Since the power dissipated in a resistor is given by the formula above.

you will see that the the smaller $R_b$ is, the greater the amount of heat generated. Whilst a dry battery will simply become warm, other batteries can generate enough heat to explode. Important: avoid shorting batteries at all costs – you could lose your life.

In the case of dry batteries, the internal resistance seems to increase the more they are used, so if you test the terminal voltage with a meter, it may be somewhere fairly close to its rated voltage. If you put the battery on load by connecting it to something such as a torch bulb, then its voltage will plummet because the load now has a resistance close to the battery's own internal resistance. The following simple circuit diagram illustrates this.

Clearly, the globe will not glow very brightly, if at all, since it only has half its rated voltage driving it. The current flowing through the circuit is 1.5 / 6 Amps, or 250 mA. The battery might still operate a jogger's radio which draws 50 mA, since in this case the voltage drop across $R_b$.is 0.05 × 3, or 0.15 Volts. The small radio would then be supplied with 1.5 - 0.15 = 1.35 Volts. It would probably work fine. It would also work for a long time in most clocks, because they draw a minute current. (That's My Nute, not Minnit).

Dry batteries have a high internal resistance, whilst lead-acid and lithium batteries have a low internal resistance. I see someone has asked the question "Can I use 10 1.5 Volt dry batteries to start a car? - after all they deliver 15 Volts".

Most motor vehicles use a starter motor having a very low resistance, requiring maybe 200 Amps over a short period of time. You would have to lower the internal resistance of your dry batteries by connecting zillions of them in parallel, and then connect the paralleled zillion batteries in series. So - the answer is theoretically yes, but practically a resounding NO.

Its an entertaining thought though!

Standard Battery Sizes

American/Popular Size

European Size

Comment

AAA

R3

Used in TV remotes etc.

AA

R6

Common battery, used in radios, cameras. Digital cameras with their high current drain have spurred on the development of high capacity types.

9 Volt/PP3

6F22

Also used in radios, calculators, test meters

Sub C

Used in radio control model cars in 7.2 Volt packs. Used in C and D size rechargeables with a surround to make them the correct dimension.

C

R14

Used in some flashlights – commonly available, but not so commonly used. Some tape recorders used to use them.

D

R20

Commonly used as a flashlight battery. Used in portable radios – especially “boom boxes”

PM9/PP9

6F22

Transistor radio battery. Still common in South Africa, as many mass-produced radios used them.

A large long version of the above PM10 /PP10

Also common in South Africa. Inexpensive long -lasting version of the above. Often fixed to the outside of radios with many elastic bands

This is a picture of some of the cells mentioned

Common Batteries

Common Battery Types

PM9 Transistor Battery

I will try to find references for the batteries used in hearing-aids and old portable radios. Here is a list:

22.5 Volt Hearing Aid Battery,

1.5 + 90 Volt Portable Radio,

1.5 + 67.5 Volt Portable radio,

1.5 + 150 Volt (To be checked) Portable Radio,

9 Volt Grid Bias Battery (Tapped)

In the projets section, you will find out how to make a battery eliminator for some of these. The 22.5 volt battery can be replaced by a Burr Brown 5 volt to 24 volt DC-DC converter.

Common Types of Primary Batteries

Dry Cells – Zinc Carbon

The dry cells of my youth in the 1950s consisted of a zinc container, which also served as the anode (-) containing a paste of ammonium chloride. A metal capped carbon rod (the cathode “+”)surrounded by a moist paste of manganese oxide was inserted into the ammonium chloride paste, and the whole assembly was sealed with coal-tar pitch or asphalt. A further cardboard tube with the manufacturer's branding was placed over the zinc container. These were simply the classic Leclanché design. Once these cells were discharged, they would leak and spoil everything.

Cross Section through a Zinc-Carbon Battery

The same cells are available today, but a”leakproof” metal jacket surrounds the whole assembly. Some modern dry cells may have a slightly different chemistry and use a zinc chloride electrolyte.

These are the least expensive battery – a D size cell costs roughly R 4.00, or US $ 0.60. Modern dry cells will supply low current devices for a long time – you may even get as much life as you would out of much more expensive alkaline batteries in things like electric clocks, calculators, simple low-power portable radios and inexpensive voltmeters.

Dry batteries used in heavy current situations rapidly drain, but soon recover. This is the origin of the word “flashlight”- early battery torches would flash and soon become dim, but would recover and shine again.

Mercury Batteries

These are no longer in general use in the standard sizes. I once bought some AA size batteries (A long, long time ago) because the instructions for my transistor radio recommended them. They had to be inserted the other way round from regular batteries. They were supposed to last longer than ordinary batteries, but I don't remember them being all that special. Mercury compounds are very toxic to the environment, so the use of it in batteries has almost been discontinued.

Alkaline Manganese

The construction of these batteries is similar to the Zinc Carbon battery, except that the outer sleeve of manganese dioxide is the positive terminal, and the anode current collector of powdered zinc is formed into a rod going down the centre of the battery. The electrolyte is potassium hydroxide. To make these batteries compatible with dry cells (you don't have to insert them the wrong way round), the manufacturers have enclosed them in a steel outer jacket connected to the negative terminal, whilst the end is connected to the anode (negative).

Cross Section through an Alkaine Manganese battery

One manufacturer claims that they last up to six times longer than regular batteries, however, the cost of a D Cell is R 27.00 (The same price that I paid for the recommended multimeter! in 2010)

I attempted to recharge these once, but they exploded making a mess of manganese dioxide everywhere.
The advantage of these cells is that they have lower internal resistance than the conventional dry cell, making them more suitable for heavy duty applications. Note that the major manufacturers are offering a higher capacity cell, for example, Duracell Ultra, Energizer Titanium and offerings from Panasonic and Ray-O-Vac.

Let's look at what batteries to use for various radios.

Example 1:

I have to choose between using AlkalineManganese or Carbon cells for a “Zenith Transoceanic” radio.
In this case Zenith Radio manual tells me that carbon batteries will last 300 hours (! - Unbelievable). Since that represents over a year's listening, I use Carbon batteries. In fact this radio has had the same Panasonic regular “D-Cells” in it for the past two years. (Very bad idea - they might leak.)

Example 2:

On the other hand, a Sony ICF-SW100 has a battery life of only 9 hours (according to the manual) using Alkaline Manganese batteries. Assuming that the life of a Carbon battery is 1/6 that of Alkaline Manganese (Duracell Ultra), I would only get about an hour and a half's use from the Carbon battery.

Example 3:

Sanyo DSB-1000 receiver that requires a 0.75 Amp supply from two D – Size AlkalineManganese batteries. Roughly what battery life can I expect, assuming the set stops working when the batteries reach 1 Volt?

Battery suppliers provide up-to-date data about their products on their web sites. This includes capacity curves for different discharge rates. Typically, they work on the amount of power drawn from the battery, so we have to work this out first. Very crudely, I'm going to assume that the radio stops working when the voltage is 1.0 volts per cell and this is the data I will use for a hypothetical battery. The power drawn from an individual cell is therefore 1.25 volts (The average of the new voltage and the voltage at end of life) multiplied by the current drawn, in this case 0.75 Amps, which gives 0.938 Watts. The following curve provides us with the battery capacity at that power drain :

This tells us that the battery has a 6.3 Amp-Hours, provided we only stick to a .75 Amp current drain. On this basis, the batteries will last 6.3/0.75 = 8.4 hours. The cost will be (in Rands) 2*54 / 8.4 = roughly 13 Rands per hour. (The batteries last roughly 8 hours in practice )

I think the conclusion we can draw is that the Sanyo and Sony radios are so expensive on batteries that they are really mains radios. In any case, to avoid the poor house, we must use rechargeable batteries of some sort.

Lithium Batteries

These deliver 3 volts or so and have very high capacity. There is also a 1.5 volt version made by “Energizer”.
They consist of a lithium anode, a separator soaked in a lithium salt and a manganese dioxide cathode, so the construction of cylindrical cells may be similar to dry cells.

In consumer applications, they are almost always used in digital photography. Even so, if you can, its cheaper to use Nickel Metal Hydride (NiMH) rechargeable batteries, with some lithium batteries in the glove box of your car for when you have forgotten your charger. Having said that, some cameras (my Samsung, for instance) don't work well off the lower voltage of the NiMH, so its better to use rechargeable Lithium-Ion batteries instead.

Important: Don't attempt to recharge lithium batteries and whatever you do, don't short them out because they can explode, but worse still they can catch fire because metallic lithium burns very readily. The energy density of these batteries is comparable to a stick of dynamite.

Zamboni Pile

You may find these referred to in very old magazine adverts – they are essentially a permanent source of static charge. They were used in night vision binoculars in World War II.

Nowadays, a low voltage battery with an electronic circuit to produce the high voltage is used for night vision binoculars. (I think!).

Rechargeable Batteries

Rechargeable Alkaline Manganese (RAM Cells)

There are a number of suppliers of these products, but in South Africa, the most prominent supplier is (or was) “Grandcell”.

They cost somewhere around double the price of regular non rechargeable batteries and are available in AA and AAA sizes. If you use them in light current applications and don't fully discharge them, you may even get a lot more than the 25 charging cycles the manufacturers claim. If they do become fully discharged, because you forgot to switch off – then the chances are slim that you will be able to recharge them.

Unfortunately I couldn't find very much data for “Grandcells” specifically, but I estimate (from what I could find) that they have about 90% of the performance of regular non-rechargeable alkalines. As a rough guide, AA cells have a capacity of around 1.8 Amp hours if you draw only 30 mA. This is about double the capacity of equivalent nickel cadmium batteries and the same as nickel metal hydride, but the voltage is higher.

If you are buying rechargeable alkaline batteries, be sure to buy a pack that contains the charger. The charger is almost free, and you won't be tempted to do something stupid, like making your own or using a NiCad charger.

RAM cells are fully charged when you purchase them, and unlike NiCad and NiMH, they retain their charge for a very long time.

Because most portable radios using AA cells draw a low current, and because RAM cells have the same voltage as dry cells, my choice is to use them for this purpose. They even work well in the SONY radio mentioned above. They also work well in doorbells and telephones – but be sure to tell other members of your household not to throw them away when they're low on charge! Like lead-acid batteries, RAM cells do not like to be fully discharged.

I have not seen these on sale for some time. They were mis-used by most people - in other words put into high current applications like digital cameras, where they would only work for 2 or 3 recharges.

Nickel Iron – NiFe batteries

These were invented by Thomas Edison in 1900. They were used in miner's cap lamps and preceded the Nickel Cadmium battery. I thought they had become quaint antiques, but apparently not so. They are still manufactured for traction applications, such as fork lift trucks. According to the manufacturer's they are very robust, have infinite tolerance for abuse and can have a lifetime of up to 30 years.

If you encounter the small ones in their grey enamel cases – hang on to them.

Nickel Metal Hydride (NiMH)

These were developed from research into hydrogen storage in the 1970's. Some metal hydrides were found to be able to store and release some 1000 times their own volume of hydrogen. Some of these alloys were subsequently found to be suitable for battery use. Replacing cadmium with a suitable alloy in the nickel cadmium battery produced a cell having some 40% more capacity for a given size.

The construction of nickel cadmium, nickel metal hydride and lithium–ion cells is similar, and consists of two long foil strips of anode and cathode material separated by a porous separator soaked in electrolyte. This is rolled into a round shape like a “swiss roll” and placed into a standard size container. The centre pin contacts the nickel positive electrode, and the outer metal hydride (or, alternatively cadmium) foil contacts the case.

These batteries were first introduced into portable computers and mobile phones, but have now given way to lithium-ion for these applications.

Provided your equipment (radio, radio/tape, camera etc.) works well off the lower voltage, these should be a good choice for a rechargeable battery solution.

Example Design - NiMH

“Boom Box” - 13 Watts consumption, designed for 6 D size cells. At full volume, this will be 13.0/6.0 = more than 2 Watts per cell. Alkaline cells will have less than 3 Amp-Hours capacity at this rate of discharge. The expected life of the batteries assuming 3.6 Watt Hours capacity would then be about 1.8 hours. I calculated the available Watt-Hours as follows: 3 optimistic amp-hours = 3 H1.2(Average) volts = 3.6 Watt-Hours per cell.

You can get D size NiMH cells with a capacity of 9 Amp-Hours or more. Unlike Alkaline cells, NiMH and NiCad cells have a very flat discharge curve, sou you can use the nominal cell voltage of 1.25. The capacity of the cell is thus 1.25 H 9 = 11.25 Watt Hours. The radio can be used for 11.25/2=5.6 Hours

The cost of a D size alkaline is R 27 and that of a D size NiMH is R 137, so you would be “breaking even” after just two charges!

Nickel Cadmium – NiCad

These are similar to the Nickel Metal Hydride (NiMH) batteries, but have a few disadvantages:

They have roughly half the capacity of NiMH, and

They are environmentally hostile.

On the other hand:

They are about half the cost of NiMH,

They also have much better self discharge characteristics than NiMH

They can be charged more times tha NiMh

Good quality NiCads can take 1000s of charges, provided you avoid overcharging, and use a decent charger.

They can deliver a higher current than NiMH – making them a better choice for power tools, model cars and aeroplanes.

After some time, these batteries may develop internal shorts circuits caused by the formation of crystals (dendrites). You can burn out these shorts with another NiCad (or, more safely, a huge capacitor charged to well above the battery voltage.) The dendrites will also reduce the capacity of the batteries, so that Nickel Cadmium batteries need reconditioning. (See “Memory Effect”).

Nickel cadmium batteries are used in toys, appliances and tools that are sold as “rechargeable”. Always try to fully discharge these batteries before recharging.

A problem with NiCad and NiMH is their high rate of self-discharge. NiMH in particular really do need to be topped-up every time you want to use them. NiCad are much better than NiMH in this respect. NiMH even self-discharge while being charged.

Example Design with NiCad

See the example for NiMH. Use a Capacity of 5.0 Amp-Hours for the D-size battery. Note that this battery may be a hard-to-find item. My local hardware sells an inexpensive 5.6 Amp-Hours unbranded D-size cell.

Consumer type NiCads and NiMH

When you read about 9Amp-hour D cells and 3.6 Amp-hour C size cells, you may think I'm mistaken. For the most part, supermarkets and hardware stores retail a “consumer type” C or D cell, that is in reality a “sub-C” cell inside a C or D size shell. Typically, “consumer” C or D size NiCads have a 1.6 Amp-hour capacity, and NiMH have a 2.6 Amp-hour capacity. They should be good deal less expensive than the “real thing”, but you may find that you can purchase high quality NiCads for less, if you shop around.

Lithium-Ion/Lithium Polymer

These have a voltage of 4.0 volts initially, gradually levelling off to about 3.6 volts. They are much lighter than NiMH or NiCad cells and have an improved amount of power for a given volume and weight (energy density).

Construction is similar to NiCad and NiMH cells, except that Lithium Cobalt Oxide is used for the positive plate and a special carbon formulation is used for the negative plate. The electrolyte is an organic solvent.

The shape of the discharge curve is similar to the NiCad and NiMH batteries.

These batteries are used for cameras, power tools, computers and mobile phones. They are supplied with a charger containing a computer chip. It is not really practical to use them as replacements for dry cells in radios – and if you did, the performance would disappoint, since you would be using one battery plus a dummy instead of two.

Lithium polymer batteries are used by radio-control enthusiats as a power source for drones and radio-controlled helicopters. Their use is somewhat beyond the scope of this web-site. They should not be over-discharged or over-charged. The radio controlled modelling community use chargers that assess the health of each cell and then apply an appropriate charge to the pack as a whole. Over discharged cells can pose a risk if you try to recharge them. Let me redirect you here for more information, as I am still coming to terms with LiPo batteries for a drone. Also, realistically, they are not going to be used by old radio enthusiasts.

They are often moulded to suit the shape of the device they power and are sometimes impossibly expensive to replace.

Lead-Acid Battery

Lead Acid batteries were originally invented by Gaston Planté in 1859. The construction is simply an arrangement of alternate plates of lead and lead oxide in an electrolyte of 30% sulphuric acid in water. The modern version is a sealed lead gel cell.

They are used in motor cars (the familiar 12 Volt Car Battery) and in most security systems and uninterruptible power supplies. They are used for traction applications, (as are as NiFE). They are also great for use with an inverter to produce 220 Volts if you have a power cut. In other words, anywhere where you need a huge current, preferably for a short time. Some military radios from World War2 era, for example, use a rotary converter which gobbles up a huge amount of power. Some early domestic radios have a vibrator power supply. These are also best powered by a lead acid battery. Vacuum tube heater supplies are often multiples of 2.1 volts, so that they can be operated from a rechargeable lead acid battery (referred to as an 'accumulator' in past times).

Lead acid batteries now come in a variety of styles and types of construction. Batteries for automotive use (SLI - Starting, Lighting and Ignition) can provide very heavy currents for a short duration, but are intolerant of the large depths of discharge experienced in emergency power applications. Deep cycle batteries for inverter use can withstand many deep charge/discharge cycles - but I couldn't even get new ones to start my car when I was stuck. There are also new chemistries that require a more complicated approach to charging.

Lead acid cells are inexpensive and robust. However, they are unsuitable replacements for dry cells, because the cell voltage is a high 2.1 to 2.2 volts per cell. Even so, Enersys technologies manufacture a D size 'Cyclon' cell with 2.5 Amp hour capacity, but it has terminals, so it will not work in a flashlight. Lead-Acid batteries have an extremely low internal resistance – the “Cyclon” battery can deliver a massive 400 amps – which practically guarantees that anything shorted across it will vapourize, accompanied by much noise, light and heat.

Sealed lead-acid batteries are normally sold as part of a system (e.g Car, Computer uninterruptible power supply, Electric fence, Gate opener, burglar alarm system and so on).

Be sure NOT to:

Short them,

Fully discharge them (always keep above 1.8 volts per cell),

Attempt to de-sulphate them using some magic advertised mixture,

Overcharge them – keep them at 2.2 to 2.3 volts per cell, and

Charge in an enclosed area – the hydrogen produced could explode.

Note that deep discharging a battery will render it permanently damaged. When you buy a battery from a hardware store - always check the cell voltage is above 2.1 volts per cell. Batteries kept in storage for a long time without charging are probably faulty.

When their life is over, please return them to a battery supplier for disposal. They are an environmental hazard and the lead can be easily recycled. I just love the warning on the the “Cyclon” battery casing:
“Battery posts, terminals and related accessories contain lead and lead compounds, chemicals known to the State of California to cause cancer and reproductive harm. Batteries also contain other chemicals known to the State of California to cause cancer.WASH HANDS AFTER USING”.

Always wash your hands after handling lead products - even after soldering.

Standard Cells

How do you know you are measuring voltage correctly ? - By comparing with a known correct voltage and adjusting your voltmeter to ngive the correct reading.

Before the advent of voltage reference chips, the way to check a voltage was with a “Weston standard cell”. This cell has a potential of 1.0190 volts, provided no appreciable current is drawn from it. (Drawing more than a few millionths of an Amp will ruin the cell.)

Weston Standard Cell

It has a cadmium negative electrode and a positive pole of mercury in contact with a paste of mercurous sulphate. The electrolyte being cadmium sulphate. The whole is assembled in a glass U tube arrangement. I can not imagine that these frail assemblies travel very well – yet I have seen them advertised on the Internet!

Fortunately, you can now purchase comparatively inexpensive semiconductor voltage references, such as Analog Devices part no. AD584, which are easy to use and will keep your voltmeter in adjustment.
Even more fortunately – there is no need to do so. The DT830B voltmeter suggested has its own built-in voltage reference. It works by comparing the voltage you are measuring with the reference voltage and displaying the result digitally. The accuracy (DC) is a “good enough” 0.5%.

Inexpensive Battery Chargers

Dry Cells and other Primary Batteries

Never attempt to recharge these – it really is false economy because they become much more prone to leak and spoil your equipment.

RAM Cells

These must only be charged with the charger supplied when you buy them. Never use these chargers for anything other than batteries of the same type and make that were supplied with the charger.

Nickel Cadmium and Nickel Metal Hydride

Battery manufacturers refer to a quantity called “C”. This is the current that the battery can supply in 1 hour. C10 (or “C10”) is the the capacity divided by 10. For example, if you have a 1800 mAh capacity NiMH battery, then C10 would be 180mA – the amount of current the battery could supply for ten hours.

To put current back into the battery again, you need to charge at C10 for a slightly longer period – 12 hours for NiCad and about 14 hours for NiMH. The figures C5 and C20 etc would be the capacity divided by 5 and 20, respectively.

If you want to charge a NiCad or NiMH battery, the simplest strategy is to charge at a constant current. For example, if you want to charge a 1800mAh battery from a 12 volt supply at 180mA(C10), then you could use the following simple circuit:

Simple NiCad/NiMH charger.

The purpose of the resistor is to provide a “reasonably constant current” to the battery. To work out its value, you need to know the charging current, the supply voltage (12 volts in this case) and make an assumption about the cell voltage under charge. I'm going to assume we have a single cell, and that its voltage will rise to 1.4 volts under charge.

Clearly, the voltage across R is going to be 12 Volts – Cell Voltage of 1.4 = 10.6 Volts. Now, assuming we have aan AA cell with 1800 mA/H capacity, then according to the manufacturer, we should charge it at 180 mA for 16 hours. From Ohm's law then:
$\displaystyle{R=V\over I}$, so $\displaystyle{R=10.6\over 0.18}$ and the value of R is thus 58.88 Ohms.
The nearest value you can buy will be 62 Ohms.

Now, we need to calculate the power dissipated in the resistor. I'm going to be really pessimistic and assume the battery is a dead short, so that all the 12 volts will be across the resistor. The power rating needed will thus be
$\displaystyle{W={V^2\over R}}$ , or $\displaystyle{144\over 62.0}$ this works out to 2.323 watts, but to be safe, use a 5 Watt resistor.

A simple battery charger as described will cost about R 100 - say US $ 10.00. There is no way that you can make one for that price, (because you need a transformer, diodes box, terminals etcetera) so if you want to make one, its purely as a “learning exercise” - which is reason enough. This type of charger is often provided with power tools for home use.

Timer controlled chargers

NiCad and NiMH batteries are fairly tolerant of overcharge, provided, you are charging at C10. However, you will get the maximum number of recharges if you avoid overcharging. Charging at C20 for a long period is also bad, since it allows crystals to grow on the battery plates. My problem is that I put them on charge, and then forget to remove them – probably you do too. If I do remove them, then they hang around and self-discharge. Unfortunately, the inexpensive chargers referred to above can easily overcharge most cells. I had some “Ever Ready” nickel cadmium batteries that I mistakenly overcharged to the extent that the pressure inside made the ends bulge out. They lasted for quite a few years after this mistreatment, even though they had leaked a little bit, so the better quality the battery, the greater the abuse it can stand.

Some chargers in the mid price range have a timer circuit built into them, so that the battery can be rapidly charged at C5 (say) and then it switches over to a trickle charge (C20) after the main charge is complete. This is fine if you fully discharge the battery between each charge. The snag is that most people can't keep track of whether the battery is fully charged, partially charged or ready for charging, so inevitably the battery will be overcharged. The other snag is that the charger has to make assumptions about the battery capacity according to its size.

After charging, check the battery for leaking electrolyte.

The “Memory Effect”

Nickel Cadmium batteries are often said to suffer from a memory effect. This was said to be a tendency for cells to reduce their capacity to a value more or less equal to the capacity that they had to deliver between charges. If you had an infrequently used cell-phone, for example, and charged it after use, then the battery would soon deliver only enough for the infrequent usage. If for some reason, you had to use the phone for a longer period – the battery would soon appear to go flat.

I do not know if this is real, or a psychological interpretation of ageing battery packs.

Charging Lead Acid and Lead Gel Cells

Because modern batteries use different chemistries, I can't recommed that you use a simple charger. A simple charger may overcharge or undercharge your battery. Either way, the life of the battery will be reduced considerably. End of charge terminal voltages can vary from 13.6 to 14.6 volts - so there's plenty of scope for messing things up.

If you have a 12 volt car battery that needs recharging from a mains chyarger, it is probably finished, or you have a faulty altenator circuit.

Sealed Lead-Acid batteries can last up to ten years and can be recharged up to 2500 times (really good batteries designed for Solar power storage), but they are not tolerant of abuse.

Charging Profiles and Multi Step Chargers

Lead Acid Batteries

In front of me, I have a Chinese manufactured 12 Volt 7 Amp-Hour battery. Somewhat unusually, it has charging information on the plastic case. For cycle use it recommends 14.4 to 15 Volts constant voltage charge, and for standby use (e.g. security systems) a constant voltage of 13.5 to 13.8 is recommended. BUT it also states that the initial charging current must be less than 2.1 Amps. "C" for this battery is 7 Amps, so they say you can begin charging at C3 and a bit - really quite high. A more normal rate of charge might be constant current of C5 or 1.5 Amps until the voltage rises to 13.8 volts, followed by a constant voltage charge of 13.8 until the current falls to C50. The termination voltage can then be held at 13.5 volts.

This is fairly typical of sealed rechargeable batteries for use in security products, but be aware that there is a difference between this simple sealed cell and (say) a "Sonnenschein" lead gel battery, which may look the same but will cost a deal more.

Lithium Polymer/Lithium Ion

Like the lead acid battery, the Lithium battery also has a charging profile similar to a lead acid cell, except the termination voltage is usually 4.1 or 4.2 volts exactly.Most of the time you will be using a charger/ battery combination, as in cell-phone, laptop or torch. Overcharging is hazardous - and when the pros get it wrong (A popular laptop, and an airliner) you can see that these batteries have to be treated with respect.

Ni MH,Cad,Fe

These batteries all charge at a constant current for a given time. After that time a really small trickle charge (C500 ?) can be given to prevent self-discharge. When the batteries are fully charged, they are at the maximum temperature and maximum voltage. The voltage will then dip slightly and the charging process can stop. Detecting the dip only works if the charging current is high enough, so the charger and battery really need to be matched to each other. Bear in mind that a NiCad AA cell may have only 500mAH capacity and a NiMH can have 2700mAH - or more.

Making your own Charger

There are several charger ICs available, all tailored to one or more type of battery. The UC3906 is an analog circuit suitable for making a lead-acid battery charger. It is used in some of the Ansmann chargers supplied by RS Electronics.It is quite expensive - so maybe make life easy and just buy the charger.

We might look at a battery charger project at some point in time - the strategy might be to use a microprocessor chip. As always, you won't save any money, but you'll learn a lot .

Manufacturer's Data Sheets

It used to be my favourite gripe that when I bought batteries, I never knew how much energy I was buying. After all, I reasoned, if I buy a kilo of mince, I kno what I'm buying and I can easily compare prices. For battery manufacturers not to disclose what I was purchasing seemed downright dishonest.

In fact, the major manufacturers do provide data sheets on their products. The problem they are faced with is that their batteries provide different amounts of energy, depending on the application.Usually, they will provide graphs of capacity vs power drain, as well as suggestions as to suitable applications.

In any event, battery capacity is probably not an exact quantity – it will vary according to the ambient temperature and many other factors, so treat the data sheets with a degree of caution.

A note on “Emergency Power”

Most hotels and “B&B”s now sport flashlights and emergency lights powered by either lead-gel or NiCads. As an experiment, I switched over to the emergency lanterns and flashlights that had been liberally placed in my room. None of them worked, except for a “Coleman” lantern that came on for about half a second.

The flashlight provided was so badly corroded, that it had become merely toxic waste.(A few days after vacating the establishment, there were severe power cuts in that area.)

Whether you are using a generator, rechargeable lights, flashlights, or whatever – be sure to test your system at least monthly to make sure it is going to work when you want it to. Its amazing how many people get shut out of their homes by their electric gates because the backup batteries have failed. The same goes for your computer uninterruptible power supply and your laptop batteries. There is no point having a “portable” computer if it's batteries do not work.

Summary

I have tried to summarize everything in the table that follows. Note that there is not a “single” solution suitable for all applications. The “~” sign means “approximately. I calculated energy density from the data for D size cells using the external dimensions of the cell except where otherwise noted.

Appendix - Peukert's Law

Peukert's equation is an empirical (i.e. determined by experiment) formula relating battery capacity to rate of discharge of a battery. It works for many battery chemistries, not just lead-acid.

You may save some time by avoiding this section and simply timing how long your battery lasts between recharges in your actual application. The reason will become clear fairly quickly.

For a one-ampere discharge rate, Peukert's law is often stated as: $C_p = {I^k }t$,

where:

$C_p $, is the capacity at a one-amp discharge rate, which must be expressed in Amp-Hour (AH).$I $, is the actual discharge current relative to 1 ampere, which is then dimensionless. $t $, is the actual time to discharge the battery, which must be expressed in hours.
$k$, is the Peukert constant, which can vary between 1.1 and 1.6 often quoted for lead-acid batteries. It gets larger with battery age.

The capacity at a one-ampere discharge rate is not usually given for practical cells, so you have to work out the formula in reverse to find the capacity at the 20 Hour rate (5 Amps for a 100 AH battery) and so on.

If I have a problem with the formula, it isn't that it is incorrect, rather it is very sensitive to the Peukert constant you choose. In addition, you don't want 100% depth of discharge. The important thing to note is that the heavier the current drain, the lower the battery capacity.

If you have an application involving heavy current drain over an extended time (as in UPS or Inverter applications), you are best advised to discuss your requirements with your battery supplier. If he can't help, I can suggest suppliers who can. Unfortunately, some UPS suppliers make fairly untenable statements about stand-by time.